A navigation system includes a display monitor, a CPU, and a camera, wherein the camera is mounted to a back side of the display monitor to form a monitor unit. One or more reference units are placed on a body while acquiring an image data set, and are tracked during a surgical operation by the monitor unit to register and correlate a position of a visual image of an exterior surface of the body with the image data set including information concerning internal structures of the body. The image data set is displayed on the display monitor superimposed in registration over the visible image of the exterior of the body with an angle of view and aperture in accordance with the actual position of the camera, whereby the display monitor displays the internal structures corresponding to the line-of-sight of the camera and the observer.
